Copper nanoparticles suspended in tin

ABSTRACT

Disclosed is a conductive ink composition and a manufacturing method thereof. The composition includes about 50 to about 99 wt % copper nanoparticles and about 1 to about 50 wt % tin. Copper nanoparticles are atomized and suspended in a tin bath, wherein the copper nanoparticles are evenly dispersed within the bath through sonification. The composition is cooled, extracted, and formed into a filament for use as a conductive ink. The ink has a resistivity of about 46.2×E-9 Ω*m to about 742.5×E-9 Ω*m. Once in filament form, the tin-copper mix will be viable for material extrusion, thus allowing for a lower cost, electrically conductive traces to be used in additive manufacturing.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made in the performance of officialduties by employees of the Department of the Navy and may bemanufactured, used and licensed by or for the United States Governmentfor any governmental purpose without payment of any royalties thereon.This invention (Navy Case 200,459) is assigned to the United StatesGovernment and is available for licensing for commercial purposes.Licensing and technical inquiries may be directed to the TechnologyTransfer Office, Naval Surface Warfare Center Crane, email:Cran_CTO@navy.mil.

FIELD OF THE INVENTION

The field of invention relates generally to printer inks for additivemanufacturing. More particularly, it pertains to a printer ink thatincludes copper nanoparticles suspended in tin for creating low costelectrical circuit traces.

BACKGROUND

Presently, a major focus in additive manufacturing relates to thedevelopment of new printer inks. With each newly developed material,advantages are created, but the material in question might only beapplicable to specific types of additive manufacturing. One area ofparticular interest is related to generating electrically conductiveinks for printing circuit traces through material extrusion. This typeof additive manufacturing is one of the more cost effective additivemanufacturing processes, however, it is hampered by limited inks. Theinks currently available either have an unacceptable resistance value orare made from silver. Silver has become the favorable ink material for alot of additive manufacturing processes due to its high conductivity andability to be printed in a foam-like state. The drawbacks of utilizingsilver are that it is susceptible to material drift from high electricalcurrents, is susceptible to oxidation, and has a high cost. Copper is apreferred conductor for additive manufacturing, but its higher meltingpoint makes it difficult to use. As such, a lack of electricallyconductive inks exhibiting low resistance values drives up the cost ofink research and development.

SUMMARY OF THE INVENTION

Disclosed is a conductive ink composition and a manufacturing methodthereof The composition uses copper nanoparticles that are atomized andsuspended in tin. The composition includes about 50 to about 99 wt %copper nanoparticles and about 1 to about 50 wt % tin. Tin is preferablyused due to it having one of the lowest melting points of all metals anda lower electrical resistance than plastics. By creating a tin bath andusing it as the medium, copper nanoparticles can be evenly dispersedwithin the bath through sonification before being formed into afilament. The composition is cooled, extracted, and formed into afilament for use as a conductive ink. The ink has a resistivity of about46.2×E-9 Ω*m to about 742.5×E-9 Ω*m.

According to an illustrative embodiment of the present disclosure, thepresent invention provides an ink utilizing copper nanoparticlessuspended in tin as a conductor for additive manufacturing.

According to a further illustrative embodiment of the presentdisclosure, the present invention provides a method of atomizing andevenly dispersing copper nanoparticles within a tin bath throughsonification before being extruded into a filament.

According to yet a further illustrative embodiment of the presentdisclosure, the present invention provides a printer ink utilizingmaterials that generate inks having lower costs and superior conductiveproperties as compared to currently known and available inks.

Additional features and advantages of the present invention will becomeapparent to those skilled in the art upon consideration of the followingdetailed description of the illustrative embodiment exemplifying thebest mode of carrying out the invention as presently perceived.

BRIEF DESCRIPTION OF THE DRAWING

The detailed description of the drawings particularly refers to theaccompanying figure in which:

FIG. 1 shows the method for manufacturing the additive manufacturingprinter ink comprising copper nanoparticles suspended in tin.

DETAILED DESCRIPTION OF THE DRAWING

The embodiments of the invention described herein are not intended to beexhaustive or to limit the invention to precise forms disclosed. Rather,the embodiments selected for description have been chosen to enable oneskilled in the art to practice the invention.

Generally, the present invention provides a conductive ink compositionand a manufacturing method thereof utilizing copper nanoparticlessuspended in tin. The manufacturing method for producing the printer inkcomprises: adding white tin to a funnel; heating the funnel to melt thewhite tin; adding copper nanoparticles to the funnel; initiatingultrasound pulses; creating a heat gradient with a higher heat locationat the upper edge of the funnel and a cooler location at the bottom ofthe funnel; and extracting a composition comprising copper nanoparticlessuspended in tin from the funnel as the composition cools.

The raw materials required for manufacturing the ink include white tin,preferably in pellet form and copper nanoparticles. Alternatively, asolution that precipitates copper nanoparticles can be used. Equipmentfor manufacturing method the ink includes a sonicator (preferably onethat works through a rod placed in a bath), a funnel with a controlvalve (preferably one that is capable of withstanding high temperaturesand that conducts heat), and a heating element (a torch, an electricheat source, or the like).

FIG. 1 shows the method for manufacturing the additive manufacturingprinter ink, comprising copper nanoparticles suspended in tin. In thefirst step, white tin is provided, preferably in a pellet form. Tin is asilvery-gray metallic element. The symbol is Sn, from the Latin stannum,and its atomic number is 50. It is usually found in the form ofcassiterite which is an oxide mineral. Tin can be obtained in a varietyof purities and in pellet form. The tin is placed in a funnel with acontrol valve. Preferably, the funnel conducts heat and is capable ofwithstanding high temperatures. Next, the funnel is heated until the tincontained therein liquefies, forming a bath. Once the tin is completelyliquefied, copper nanoparticles are added to the funnel. Preferably,copper nanoparticles are obtained and used directly, however, if coppernanoparticles must be formed first from a solution, it is desirable toreact the solution to produce a precipitate and then to vacuum oven offthe remaining liquid. The copper nanoparticles are atomized and becomesuspended within the tin.

The next step is to add the sonicator and to begin ultrasound pulses.Sonication causes the copper nanoparticles to be evenly dispersed withinthe tin bath before they are extruded into a filament. Ultrasound is atechnique used in metallurgy to produce stronger, lighter metal meltsand as a method of cleaning. When used for cleaning, the ultrasoundtravels through a liquid medium in which cavitation removes contaminantsfrom the object. The cavitation created by the ultrasound can varydepending on the medium, the frequency, and the strength of the signal.By carefully tuning these factors (as described in further detailbelow), the microcavitation will prevent the bath from boiling whilethoroughly mixing the copper and tin. A bath of tin using ultrasound candistribute the copper nanoparticles while also degassing the suspension.

After sonication is complete, a single filament can be drawn by slowlydraining through a cooling chamber. The heating element is removed oradjusted so that heat is focused on the upper edge of the funnel, whichcreates a heat gradient with a higher temperature at the upper edge ofthe funnel and a cooler temperature at the bottom of the funnel. Oncethe heat gradient is established, the composition is slowly removed fromthe funnel as it cools. The heat gradient enables the composition to beextruded into a filament as it is removed. In the preferred embodiment,a cool bath is recommended to accelerate the cooling process.

The above described method will be addressed in more detail through thefollowing example, which assumes a 1 kilogram sample for the finalsuspension for all calculations. Table 1 illustrates a calculation usingthe resistivity of both tin and copper to determine the overallresistivity based on what percentage of the final volume each makes up.Based on the calculations, a mixture of 99% copper to 1% tin would yieldresults almost exactly the same as copper alone, while 85% copper to 15%tin would yield about double the resistivity. These calculations rely ona theoretical volume based on the density of the material and the weightof each contributing component.

TABLE 1 shows the resistivity by weight of tin-copper alloys. totalDensity resistivity weight Volume resistivity (g/cm{circumflex over( )}3) (Ω*m) cu/sn % in g (cm{circumflex over ( )}3) volume % (Ω*m)Copper 8.94  16.8E−9 90% Cu 900 100.671 0.880 27.9E−9 Tin 7.31 110.0E−910% Sn 100 13.680 0.120 99% Cu 990 110.738 0.968 17.6E−9 1% Sn 10 1.3680.012 85% Cu 850 95.078 0.831 33.7E−9 15% Sn 150 20.520 0.179 50% Cu 50055.928 0.489 74.0E−9 50% Sn 500 68.399 0.598 75% Cu 750 83.893 0.73445.2E−9 25% Sn 250 34.200 0.299

A basic model that takes into account the atomic structures can bederived using the Drude model and the crystal structure of both tin andcopper. Equation 1 calculates resistivity using a combination of theMatthiessen Rule and Nordheim Rule.

ρ=ρ_(matrix)+CX(1−X)   Equation 1

The Nordheim coeffiecent (C) is an empirically derived resistance valuefor copper-tin scatter. A Nordheim coeffiecent for solid copper as asolvent was used. To account for the use of copper nanoparticles, theresistivity coefficient was adjusted accordingly.

Table 2 shows the calculation based on the Drude model with differentpercentages of tin and copper yielding a final total resistivity shownon the right. As shown, the content of copper nanoparticles is about 50to about 99 wt %, and the content of tin is about 1 to about 50 wt %.Currently, the best material extrusion conductive ink is Voxel8's silverink with a bulk electrical resistivity around 3.0×E-7 Ω*m. All of thetotal resistivity values are shown in Table 2 and show that theinventive conductive ink composition has a resistivity of about 46.2×E-9Ω*m to about 742.5×E-9 Ω*m. These values fall close to the value of thesilver ink with some being theoretically better.

TABLE 2 Drude model of total resistivity. Total Density NordheimResistivity (g/cm{circumflex over ( )}3) Coefficient cu/sn % % (Ω*m)Copper 8.94  2.9E−6 90% Cu 0.9 278.5E−9 Tin 7.31 resistivity 10% Sn 0.1matrix (Ω*m) 99% Cu 0.99  46.2E−9 17.5E−9 1% Sn 0.01 85% Cu 0.85387.3E−9 15% Sn 0.15 50% Cu 0.5 742.5E−9 50% Sn 0.5 75% Cu 0.75 561.3E−925% Sn 0.25

In order to properly mix and degas the tin/copper mixture, ultrasoundmust be applied to the mixture. Ultrasound is also used in the processof atomizing particles in a solution. Starting with the Navier-Stokesequations and assuming spherical symmetry of cavitation bubbles in aninfinite body of liquid the Rayleigh-Plesset equation (Equation 2) canbe derived.

$\begin{matrix}{\frac{{P_{B}(t)} - {P_{\infty}(t)}}{\rho_{L}} = {{R\frac{d^{2}R}{{dt}^{2}}} + {\frac{3}{2}( \frac{dR}{dt} )^{2}} + {\frac{4\nu_{L}}{R}\frac{dR}{dt}} + \frac{2S}{\rho_{L}R}}} & {{Equation}2}\end{matrix}$

Equation 2 models how cavitation occurs in a liquid but it can also beused to derive Equation 3, which is the density of gas flow into thebubble as a function of time. By applying ultrasound to these bubbles, apressure gradient can be formed within the bubble causing a rippleeffect. The pressure gradient can pull gas from the cavitation allowingthe gasses to flow from the liquid.

(R/3κT)∂t+P∂R/(∂tκT)=i(t)   Equation 3

Rearranging Equation 3 as a function of the unknown radii (S(r)=4πr2)results in Equation 5. Using the assumptions made by Eskin, let thesurface area be the unknown in the system of equations for bubbledynamics Equation 4 and density of gas flow into the pulsating bubbleEquation 5.

∂² S/∂i ²=−(1/4)S(∂S/∂t)²+2π/ρ[P(t)−P _(Q)−P _(S)sin ωt]−4δπ^(3/2) /S^(1/2)−4μŚ/S   Equation 4

3S ^(3/2) /√πκT(∂P/∂t)+PS ^(2/2)(2 √πκT(∂S/∂t)=i   Equation 5: whereinP_(S) is the amplitude of sound pressure; p is the density of tin; δ isthe surface tension of the tin; μ is the viscosity of the tin; D is thediffusion coefficient; C₀ is the content of hydrogen in liquid; and T isthe period of bubble pulsation

Equations 4 and 5 were solved using numerical methods with intialconditions of S(0)=S₀, P(0)=P₀ and S′(0)=0. The result indicated thatthe ideal ultrasound pressure ranges from approximately 15MPa to 25MPawith pressures exceeding 25MPa being subject to dramatic changes inpulse exposure time. Exposure time of the tin/copper bath ranges fromabout 60 seconds to 150 seconds.

Although the invention has been described in detail with reference tocertain preferred embodiments, variations and modifications exist withinthe spirit and scope of the invention as described and defined in thefollowing claims.

1. A composition comprising: a conductive additive manufacturing printerink having a resistivity of about 46.2×E-9 Ω*m to about 742.5×E-9 Ω*m.2. A conductive ink composition comprising: atomized coppernanoparticles suspended in white tin; wherein the wt % of said atomizedcopper nanoparticles and said white tin is selected to produce aconductive ink composition a having resistivity of about 46.2×E-9 Ω* mto about 742.5×E-9 Ω*m; wherein said resistivity is calculated using acombination of the Matthiessen Rule and Nordheim Ruleρ=ρ_(matrix)+CX(1−X).
 3. The composition of claim 2, wherein the contentof said copper nanoparticles is about 90 wt % and the content of saidtin is about 10 wt %; and wherein said conductive ink composition has aresistivity of about 278.5×E-9 Ω*m.
 4. The composition of claim 2,wherein the content of said copper nanoparticles is about 99 wt % andthe content of said tin is about 1 wt %; and wherein said conductive inkcomposition has a resistivity of about 46.2×E-9 Ω*m.
 5. The compositionof claim 2, wherein the content of said copper nanoparticles is about 85wt % and the content of said tin is about 15 wt %; and wherein saidconductive ink composition has a resistivity of about 387.3×E-9 Ω*m. 6.The composition of claim 2, wherein the content of said coppernanoparticles is about 50 wt % and the content of said tin is about 50wt %; and wherein said conductive ink composition has a resistivity ofabout 742.5×E-9 Ω*m.
 7. The composition of claim 2, wherein the contentof said copper nanoparticles is about 75 wt % and the content of saidtin is about 25 wt %; and wherein said conductive ink composition has aresistivity of about 561.3×E-9 Ω*m.
 8. The composition of claim 1,wherein said composition is extruded into a filament.
 9. The compositionof claim 2, wherein said composition is extruded into a filament.
 10. Amethod of manufacturing copper nanoparticles suspended in tincomprising: adding white tin to a funnel; heating said funnel to meltsaid white tin; adding copper nanoparticles to said funnel; initiatingultrasound pulses; creating a heat gradient; and extracting acomposition comprising copper nanoparticles suspended in tin.
 11. Themethod of claim 10, wherein said composition is extruded into afilament.
 12. The method of claim 10, wherein the content of said coppernanoparticles is about 50 to about 99 wt % and the content of said tinis about 1 to about 50 wt %; and wherein said conductive ink compositionhas a resistivity of about 46.2×E-9 Ω*m to about 742.5×E-9 Ω*m.
 13. Themethod of claim 12, wherein, wherein the content of said coppernanoparticles is about 99 wt % and the content of said tin is about 1 wt%; and wherein said conductive ink composition has a resistivity ofabout 46.2×E-9 Ω*m.
 14. The method of claim 12, wherein the content ofsaid copper nanoparticles is about 85 wt % and the content of said tinis about 15 wt %; and wherein said conductive ink composition has aresistivity of about 387.3×E-9 Ω*m.
 15. The method of claim 12, whereinthe content of said copper nanoparticles is about 50 wt % and thecontent of said tin is about 50 wt %; and wherein said conductive inkcomposition has a resistivity of about 742.5×E-9 Ω*m.
 16. The method ofclaim 12, wherein the content of said copper nanoparticles is about 75wt % and the content of said tin is about 25 wt %; and wherein saidconductive ink composition has a resistivity of about 561.3×E-9 Ω*m.